Vehicles may be fitted with evaporative emission control systems to reduce the release of fuel vapors to the atmosphere. For example, vaporized hydrocarbons (HCs) from a fuel tank may be stored in a fuel vapor canister packed with an adsorbent which adsorbs and stores the fuel vapors. At a later time, when the engine is in operation, the evaporative emission control system allows the fuel vapors to be purged into the engine intake manifold from the fuel vapor canister. The fuel vapors are then consumed during combustion.
In one example described by Covert et al. in U.S. Pat. No. 5,878,729, a fuel vapor canister includes a plurality of inlet ports and purge ports regulated by respective valves. During operation of the engine, the purge valves and the air inlet valves are opened to supply a negative pressure from an engine air induction passage to within the canister. As a result of the supply of the vacuum, fuel vapor is purged to the intake manifold of the engine from the fuel vapor canister.
However, the inventors herein have recognized issues with the above approach. For example, in engine applications that operate with low vacuum air induction, or near atmospheric pressure (as measured post throttle body in the engine's intake manifold), the small amount of vacuum may not be enough to sufficiently purge the fuel vapor canister. More particularly, in hybrid electric vehicle (HEV) applications, the engine run time may be shorter than the amount of time it takes to purge the fuel vapor canister with low vacuum. As such, if the canister is not completely purged, exhaust hydrocarbons may slip into the atmosphere, degrading exhaust emissions and making the vehicle emissions non-compliant. In addition, the low vacuum may increase the engine operation time required to purge the fuel vapor canister. The unintended increase in engine run time for the hybrid vehicle can degrade vehicle fuel economy.
Thus, in one example, some of the above issues may be at least partly addressed by a method for operating an engine comprising displacing an amount of unthrottled intake air with air received from a fuel system canister. In this way, a fuel system canister can be purged even when there is low vacuum induction in an engine.
For example, a fuel system canister may be purged using intake air that is substantially at atmospheric conditions. The canister may be a multi-port canister having a plurality of intake ports or vents, as well as a plurality of purge ports. When purging conditions are met, a vent control valve may be opened to enable atmospheric air to enter the canister through the multiple vents and desorb stored fuel vapors from the canister. The fuel vapors may then be purged to an engine intake upon passage through the multiple purge ports by opening a purge valve. A diverter valve coupled between the purge line and an intake passage may be opened so that the fuel vapors can be received upstream of an intake throttle. In particular, the opening of the diverter valve may be adjusted so that an amount of intake air received in the engine intake is displaced by the ingested fuel vapors. For example, as the amount of fuel vapors ingested increases, an amount of intake air may be correspondingly decreased.
In this way, a purge flow is created by redirecting an amount of incoming engine air mass from an engine's air cleaner to enter from a fuel vapor canister. By using air that is substantially at atmospheric pressure to purge a canister, a vacuum requirement for purging is reduced. By purging multiple regions of the canister simultaneously, a time required to completely purge the canister is lowered. By better enabling canister purging to be completed, the likelihood of attaining zero bleed emissions from the canister is increased. By displacing an amount of intake air directed to an engine with fuel vapors received from a canister, and by mixing intake air with the fuel vapors upstream of an intake throttle before delivering the mixture to an intake manifold, a combustion air-to-fuel ratio can be maintained. Overall, exhaust emissions and emissions compliance may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.